The present invention generally relates to digital communications and, more specifically, to a method and apparatus for cancellation of inter-carrier interference (ICI) in a communication system such as an orthogonal frequency division multiplexing (OFDM) system.
In wireless communication, a signal may be converted into an electro-magnetic wave and transmitted through a physical channel such as air or other medium to a receiving end. Due to channel effects caused by multi-path reflection, diffraction or refraction, a received signal may suffer from interference. Moreover, a multi-path may become a frequency selective fading channel. For a single carrier modulation system, a receiver may be designed with a complicated time-domain equalizer so as to equalize the channel effect resulting from, for example, the multi-path reflection. However, in an OFDM system using multi-carrier modulation technology, by applying a guard interval (GI) in transmitting an OFDM symbol, the multi-path reflection channel effect may be significantly reduced. As a result, a receiver of the OFDM system may only need a simple equalizer such as a one-tap equalizer to equalize constructive interference or destructive interference from the channel. Therefore, OFDM has become an important technology in wired communications such as asymmetric digital subscriber line (ADSL) and power line communication (PLC) and in wireless communications such as wireless local area network (WLAN) based on the IEEE 802.11a/g/n standards, digital video broadcasting-terrestrial (DVB-T), digital video broadcasting-handheld (DVB-H) and digital audio broadcasting (DAB). Furthermore, the OFDM technology may also be applicable to the fourth generation (4G) personal mobile communications.
Some standards for the OFDM system, for example, the DVB-H and IEEE 802.16e standards, may require desirable reception performance of a receiver when moving at a relatively high speed. However, as a receiver is mobile with respected to a transmitter in an OFDM system, the channel impulse response during an OFDM symbol may not remain the same, which may result in a time-selective fading channel. Meanwhile, Doppler effect may occur, resulting in a frequency offset that is approximately one Doppler frequency (fc) shift from the carrier frequency. Moreover, the frequency offset may be significant in an OFDM multi-carrier modulation system and cause an inter-carrier interference (ICI) effect, which in turn may deteriorate the orthogonality of OFDM signals and incur an “error floor” phenomenon.
FIG. 1 is a plot illustrating the error floor phenomenon. The plot may be made from the results of a simulation on the bit-error rate (BER) at various signal-to-noise power ratios (SNR) of a receiver in an OFDM system under the influence of ICI effect caused by Doppler effect, wherein the SNR in an multi-level quadrature amplitude modulation (M-QAM) modulation scheme can be defined as log2M×Eb/NO where Eb/NO is the ratio of the energy of one bit Eb to the noise power spectrum density NO. Referring to FIG. 1, a curve 102 represents a first condition near inter-carrier interference free (ICI-free), and curves 104 represent a second condition that the error floor phenomenon occurs, where the BER does not decrease as the SNR increases. By comparison, the BER on the curve 102 may decrease as the SNR increases and the smallest BER and largest SNR may be found on the curve 102. It may lead to a conclusion that the ICI effect caused by Doppler effect may deteriorate the performance of the OFDM receiver.
FIG. 2 is a diagram illustrating an OFDM sequence including OFDM symbols 202, 204 and 206 in an OFDM system. Referring to FIG. 2, in order to enhance the performance of the OFDM system and alleviate the multi-path reflection channel effect, a transmitter of the OFDM system may periodically duplicate a section 220 of the useful symbol 210 and prefix the duplicate, i.e., a guard interval 208, to the useful symbol 210 in time domain to form a complete OFDM symbol such as the OFDM symbol 202. The OFDM symbol 202 may therefore include the guard interval (GI) 208 with a length Tg and the useful symbol 210 with a length Tu. When the guard interval 208 is longer than a maximum delay 214 with a length τmax (i.e. Tg>τmax), an inter-symbol interference free region (ISI-free region) 212 may exist. The OFDM symbol 202 may then be transmitted into the channel. When the OFDM symbol 202 is received, the receiver may eliminate the inter-symbol interference due to the multi-path reflection channel effect by dropping the whole guard interval 208 directly. The useful symbol 210 may then be extracted and the influence caused by the channel may be compensated with a one-tap equalizer in order to estimate the data of the OFDM symbol 202.
In an environment wherein a receiver is moving at a relatively high speed, the length τmax of the maximum delay 214 may be much shorter than the length Tg of the guard interval 208. In some applications, for example, the length Tg may be as long as a quarter (¼) of the length Tu of the useful symbol 210. The ISI-free region 212 within the guard interval 208 may be used to reconstruct the transmitted signal and alleviate the ICI effect. Some prior art techniques have been proposed to alleviate the ICI effect based on the use of an ISI-free region. An example of the prior art techniques may be found in a paper, entitled “Improving an OFDM Reception Using an Adaptive Nyquist Windowing,” by C. Muschallik, IEEE trans. Consumer Electron, vol. 42, no. 3, pp. 259-269 (hereinafter referred to as “the Muschallik”), Aug. 1996. Muschallik disclosed an OFDM receiver that employs a 2N-point FFT module. Another example of the prior art techniques may be found in a paper, entitled “Receiver Windowing for Reduction of ICI in OFDM Systems with Carrier Frequency Offset,” by N. C. Beaulieu and P. Tan, 2005 IEEE Globecom proceedings, vol. 5, pp. 2680-2684, December 2005 (hereinafter referred to as “the Beaulieu”). Beaulieu and Tan also disclosed an OFDM receiver that employs a 2N-point FFT module. The above-mentioned 2N-point FFT modules may be more complicated and costly than an N-point FFT module. However, these OFDM receivers may not have achieved significant improvement in performance.
Still another example of the prior art techniques may be found in U.S. Pat. No. 5,357,502A to Castelain et al, entitled “Device for the Reception of Digital Data Time Frequency Interlacing, Notably for Radio Broadcasting at High Bit Rate towards Mobile Receivers with Nyquist Temporal Window” (hereinafter referred to as “the Castelain). Castelain disclosed a method of using a Nyquist temporal window for solving a time frequency interlacing problem. In Castelain, windowing coefficients may be obtained based on “Raised-Cosine Coefficients.” The use of raised-cosine coefficients, as can be seen in Muschallik's design. Furthermore, U.S. Patent Application Publication No. 2006/0239367A1 by L. Wilhelmsson and M. Faulkner, entitled “Low Complexity Inter-Carrier Interference Cancellations” (hereinafter referred to as “the Wilhelmsson”), disclosed a method for reducing or eliminating the ICI effect. Although both the Castelain's and Wilhelmsson's methods may be simple because they need only an N-point FFT, windowing coefficients thus produced may not lead to a significant improvement in reducing or eliminating the ICI effect.
It may therefore be desirable to have a method and apparatus capable of forming windowing that may be less complicated and more cost efficient than the prior art techniques.